Water is a universal solvent, constantly dissolving small amounts of minerals from the environment it contacts; one of the most common is calcium. The presence of calcium in drinking water is a natural consequence of the hydrological cycle and a feature of many water supplies. For most people, this mineral content leads to the question of whether it is safe to drink. Calcium in water is generally not considered a health concern, and its effects relate primarily to its concentration and the physical impact it has on household systems.
How Calcium Enters Water and How It Is Measured
Calcium enters the water supply through a geological process known as mineral dissolution. As rainwater seeps through the earth, it becomes slightly acidic by absorbing atmospheric carbon dioxide, forming a weak carbonic acid. This mildly acidic water moves through underground rock formations, particularly those rich in calcium-bearing minerals such as limestone, chalk, and gypsum. The acid dissolves the calcium, which releases positively charged calcium ions (Ca²⁺) into the water supply.
The concentration of these dissolved ions, along with magnesium, determines the level of “water hardness.” Hardness is quantified using two common units: parts per million (ppm) or grains per gallon (gpg). The measurement is typically expressed as the concentration of calcium carbonate (CaCO₃).
Water supplies are classified based on these measured concentrations, ranging from soft to very hard. Soft water generally contains less than 60 ppm, while moderately hard water falls between 60 and 120 ppm. Water is considered hard when concentrations exceed 120 ppm, and very hard when it is above 180 ppm. These levels fluctuate widely across regions, depending on the local geology that the water source flows through.
Nutritional and Safety Aspects of Waterborne Calcium
For the vast majority of people, the calcium naturally present in drinking water is safe and provides a small nutritional benefit. Calcium is an essential dietary mineral, playing a role in bone strength, nerve function, and muscle contraction. The calcium ions dissolved in water are highly bioavailable, meaning the body can absorb and utilize them effectively, comparable to the calcium found in dairy products.
Drinking water can contribute an estimated 5 to 20% of the average person’s daily recommended calcium intake, depending on the water source’s hardness and the volume consumed. Regulatory bodies do not set a maximum contaminant level for calcium, treating its presence as an aesthetic or operational concern rather than a health hazard.
There is no general risk of consuming too much calcium, or hypercalcemia, from drinking water alone, as water is a minor source compared to dietary intake. The body has a tightly regulated system for absorbing calcium, which prevents over-accumulation in most cases. However, for individuals with specific pre-existing conditions, such as a history of recurrent kidney stones, doctors may advise monitoring total calcium intake from all sources, including very hard water.
Practical Effects of Calcium on Home Water Quality
The primary consequences of calcium in water are not related to health but rather to the physical effects it has on household plumbing and appliances. High concentrations of calcium ions lead to the formation of limescale, a hard, chalky deposit of calcium carbonate. This scale builds up inside pipes, reducing water flow and potentially shortening the lifespan of the plumbing system.
Calcium is especially problematic in appliances that heat water, such as water heaters, dishwashers, and coffee makers. Limescale acts as an insulator on heating elements, forcing the appliance to use more energy to reach the desired temperature. This decreases efficiency, raises utility costs, and causes aesthetic issues like cloudy spots on glassware and white buildup on faucets and showerheads.
Calcium also significantly affects how soap and detergents perform in the home. The calcium ions react with the fatty acids in soap, forming an insoluble precipitate known as soap scum. This reaction prevents soap from lathering effectively, requiring more product for cleaning, bathing, and laundry. High calcium levels can sometimes affect the water’s palatability, giving it a slightly metallic or earthy flavor.